This invention relates to a method and an apparatus for measuring a three-dimensional position of a surface of a lengthwise object such as a pipe having a uniform cross section from a corresponding two-dimensional observed image of the object surface to measure, for example, the size of a defect in the surface.
Conventionally, methods described below have been used to measure the length of a defect existing in an inner surface of a pipe.
In one conventional method, a reference object the length of which is known is placed in a pipe, and the reference object and a defect are compared with each other in an image to measure the length of the defect.
In other conventional methods, a chart is projected at a distance (base length) from an observation optical system, and three-dimensional coordinates are calculated on the basis on a misalignment of the chart in an observed image, or a patten is projected as disclosed in Japanese Utility Model Laid-Open No.57-29806 to calculate three-dimensional coordinates. In a method disclosed in Japanese Patent Laid-Open No.63-140903, a light sectioning line is projected to measure a position on a cylindrical object.
Thus, measurement of a defect in a pipe or a similar structure has been performed by using a reference object with respect to the measured object, projecting a chart or using an auxiliary means. That is, three-dimensional coordinates are not calculated from an observed image alone.
There are therefore problems relating to the need for a means for projecting a chart or an auxiliary means, and a problem of apparatuses capable of such measurement being limited. There is a need for a system enabling existing apparatuses having no such means to measure length and other quantities. It is very convenient if there is a system capable of measuring a target by obtaining variables, other than known constants, required at the time of measurement from an observed image, in other words, capable of measurement using no auxiliary measuring instrument.